In order to manufacture various components such as airplane wings, it is known practice, for example from document U.S. Pat. No. 4,842,684 in the name of the Applicant Company, to use a robot or a machine fitted with a draping head to lay automatically onto a layup tool or mold a composite tape made of elements of fiber (carbon, glass, an aramid resin for use in the fabrication of synthetic fibers sold under the trademark KEVLAR®, etc.) which are stuck together by impregnating with a thermosetting resin (the elements are then generally supported on a backing tape made of paper or plastic film) or a thermoplastic resin. The composite tape is paid out from a reel to pass over the draping head where the tape of prepreg fibers is detached from the backing tape, the latter being returned to a winding mandrel, while the prepreg is applied to the mold or to the previous layers already laid up, by a compaction member generally consisting of a roller or an applicator shoe connected to the draping head. Suitable members guide the prepreg to the layup zone, downstream of the strip circuit, and separate the prepreg from its backing as close as possible to the layup member. The draping head is mounted with an ability to move along several axes (generally five axes or six axes) so as to conform to the increasingly complex shapes of the components that have to be produced using this technique. This head moves at very high speed, for example at 1 m/s.
Bearing in mind the shapes of the components produced, and notably of their edges, it is necessary not only to lay up what is known as “full width” lengths of tape, with four sides, but also lengths of various shapes, obtained by complex cutting-out of the tape.
For through-cuts of simple shapes, use is made of a single-phase layup process which involves the in situ cutting of the strips that are to be laid up and immediately laying them using the same machine. A mechanical or ultrasonic knife cuts the prepreg tape directly on its backing paper without cutting through this paper which will then be wound back onto the mandrel after peeling.
For complex shapes of strip to be cut out, use is made of a two-phase process whereby the cutting is done upstream in the process, using a specialized first machine, and whereby the successive precut portions are left on the initial backing tape or placed between two protectors and wound onto a cassette which is then installed on the draping head.
Whether it is the single-phase process or the two-phase process that is used, a draping head is used for laying up the prepreg tapes and this head may be a single or double head: the latter comprises two strip-laying circuits which downstream end in a layup zone at the bottom end of the head. Documents FR2888156, FR2894510 and FR2949378 in the name of the Applicant Company disclose exemplary embodiments of such double draping heads, generally with two compacting members, in this instance rollers, one per circuit, which may be of different widths. These two rollers are generally very close together, leaving only a narrow space between them, typically a space of under two centimeters.
It is important constantly to check the quality of the laying-up of the strip laid by the draping head and notably to verify that the various portions of strip do not overlap but, on the other hand, do actually have the requisite clearance between them.
This checking can be done purely visually, using measuring magnifying glasses. However, it is very difficult because the strips of carbon are black and the clearances do not show up well; the components to be examined may be very big and large, forcing the operator tasked with making the visual inspection to climb up onto the components with the risk of damaging them; and finally, the number of clearances to be checked is extremely large because there may for example be 10,000 complex strips to check in one component.
It is therefore desirable to find a method and a device for checking the clearance automatically.
Attempts have been made at checking the clearances using suitable illumination of the surface and observation thereof using a camera the images (contrasts) of which are analyzed, but experience has shown that the results are of low reliability, this not exceeding 70%.
Moreover, it has been found that the variations in distance between the camera and the surface to be observed can lead to difficulties in focusing the checking camera entailing corrections for the measured height, taking this distance into consideration.
Furthermore, the ideal location at which to perform the check, right in the region of the two compacting rollers in the case of a double draping head, is very tight and does not leave a lot of space in which to install inspection equipment.
Document US 2007/271064 discloses a draping subassembly.
Document US 2007/229805 discloses a checking device included within a draping head.
However, a device and a method that improve the reliability of the existing systems and that are suited to being installed in a tight space within the draping device have yet to be found.
It is therefore an object of the invention to provide a method and an on-board device for checking the clearances between strips or, more generally, for checking the surface of the laid strips, which overcome these difficulties.